Cooling System Modifications... 2

Transcription

1 COOLING SYSTEMS Cooling System Modifications... 2 Thermal Efficiency... 2 Cooling System Goals... 2 Nucleate Cooling Phase... 2 Types of Coolant... 2 System Pressure... 3 Stock Cooling Systems... 3 Mechanical Water Pumps... 3 Electric Water Pumps... 3 Coolant Flow... 4 Basic Flow Modifications... 4 Radical Modifications... 5 Reverse Flow Systems... 5 Cooling Fans... 6 Stock Clutch Fans... 6 Flex Fans... 6 Electric Fans... 6 Fan Shrouds... 7 Radiators... 8 Copper Brass vs. Aluminum... 8 Cross Flow vs. Down Flow... 8 Single vs. Dual Pass... 8

2 COOLING SYSTEM MODIFICATIONS THERMAL EFFICIENCY Before we go into what a cooling system does and how to modify it, you must first understand what the engine does. Plain and simple, an engine burns a fuel to generate heat energy and transforms that heat energy into mechanical energy. Any heat generated that does not get used to make power is wasted energy. How well an engine converts the heat it generates into mechanical energy is known as its thermal efficiency. The cooling system takes heat from the engine, heat that ideally could have made power, so the cooling system actually takes power from the engine. It is a necessary evil, without a cooling system, the engine will overheat and the internal parts will have a very short life. A cooling system also reduces the chances of detonation. With new cooling systems and coolants, it is possible to run today's engines hotter, which increases thermal efficiency. If you take less heat away from the engine, there will be more energy available to make power. Any heat that is radiated off the engine and out the exhaust system is also wasted heat energy that did not get used to make power, which reduces thermal efficiency. The average engine has only a 25-30% thermal efficiency, so 70-75% of the heat generated never gets used to make power. An average 250hp gasoline engine is actually burning enough fuel to make about 1000 hp, making it a very inefficient machine. COOLING SYSTEM GOALS Most people seem to think that all a cooling system needs to do is keep the engine from overheating. But what is not realized is that if the engine runs too cool, thermal efficiency is lost and power is reduced. Many will argue that an engine has more power when it is cold, but that is only due to the fact that the intake air is colder and denser, actual BSFC is higher. Remember that an engines whole job is turn heat into mechanical energy. Running the engine as hot as possible, limited by the detonation limit, will increase power and provide a lower BSFC. If steam pockets will form, detonation will limit power. Most of today's high output street motors using a water/ethylene glycol mixture will be limited to about 200 F before detonation becomes a problem (unless other steps are taken). Another goal of modifying the cooling system is to even out the temperatures of the whole engine, which is not easy to do. All it takes is one hotter cylinder to run into detonation to limit the engines power. It only takes 1 cylinder to limit all of them. Most high performance engines are close to detonation to begin with, so a good cooling system is a must. NUCLEATE COOLING PHASE As coolant flows through the system it absorbs heat from the engine parts that it comes in contact with. As it does this some of the coolant will boil and form tiny steam bubbles (absorbing a lot of heat in the process) on the internal engine surfaces. When these bubbles get larger they become a flow restriction and the flowing fluid pushes them away from the surface and that process starts over again. The process is called the Nucleate Cooling Phase. When the coolant boiling point is too low or the flow rate is too slow, these bubbles can become too large and form steam pockets that insulate that surface from being cooled. This usually happens around the combustion chambers, the hottest parts of the engine. Once the steam pocket forms, the surface will rise in temperature, even though the average coolant temperature is normal, and cause that part to overheat, which can cause detonation and / or other problems. TYPES OF COOLANT 2

3 I'm sure that you've read or heard somewhere before that water is the best coolant. This is true as far as being able to absorb heat for a given flow rate, water does do that the best. Water also boils at a lower temperature than other coolants and can develop steam pockets easier, so it is not the best coolant in that respect. A water / ethylene glycol mixture will boil at a higher temp and resist steam pockets better than plain water, the down fall is that it has to have a higher flow rate, but that is easy to accomplish. The 3rd common form of coolant is propylene glycol, which has the highest boiling point and can run higher than 250 F (average temperature as seen on a gauge) without forming steam pockets, but it must flow at more than twice the speed of a water / ethylene glycol mixture (which means major changes to most cooling systems). SYSTEM PRESSURE The pressure in the block is higher than the radiator pressure; this is because the pump is building pressure due to the thermostat being a restriction. This pressure raises the boiling point of the coolant and reduces the chance of steam pockets, so never run with out a thermostat (or some form of restriction). The radiator cap will usually hold psi, if the radiator holds the system at 15 psi, the boiling point of plain water will be raised to 250 F. The water pump can then make an additional psi in the engine and bring that boiling point close to 300 F. So as you can see, pressure is important. STOCK COOLING SYSTEMS Most stock cooling systems pull coolant from the radiator and push it through the each bank of the block; it then goes up through holes in the head gasket(s) to the heads and out the front of the heads to a common exit point. This ok for a stock engine that has no problems with detonation, but the cooling is very uneven. The front cylinders will run coolest and the front combustion chambers will run the hottest. Most stock pumps will also favor one bank. The stock pump used on a small-block Chevy for instance will always favor the passenger side bank. This means that cylinder 2, 4, 6 & 8 get more flow, so the 1, 3, 5, & 7 bank runs hotter. With the center exhaust ports right next to each other, you can see that combustion chambers 3 and 5 will run the hottest; it is in these two cylinders that detonation will usually first start. It seems a little backward to start the coolant at the block instead of the heads; it would make more sense to bring the coolest coolant to the hottest parts first. This type of reverse flow system has been tried with much success, but it is harder to get it working properly and not worth it for car companies to research when the stock system worked good enough on a stock engine. MECHANICAL WATER PUMPS As I said before, stock pumps rarely flow evenly between banks. On the small-block Chevy you can restrict 1/2 of the block inlet to the even cylinder bank to get more even flow, but the better solution is to use an aftermarket high volume pump that has worked out such problems. Stock pumps have a stamped steel impeller and tend to cavitate easily when turned more than 6000 rpm, so overdriving the stock pump offers little to no advantages and can actually aggravate any cooling problems. Most aftermarket pumps will use a cast iron or an aluminum impeller that better resists cavitation. Weiand, Howard Stewart and Milidon make very good water pumps for most popular applications, which improve flow, resist cavitation better, and require less power to drive than stock pumps. ELECTRIC WATER PUMPS Many aftermarket companies offer electric water pumps. Many of these pumps do not flow well or build sufficient pressure in the block. They are only good for limited drag racing use, and when used they need a high pressure cap to help prevent steam pockets. If you are considering an electric pump, don t settle for anything that flows less than gallons per hour and that may not be enough. Many of these pumps flow less than 20gph and cannot keep up with the demands of street driving. 3

4 Even a stock mechanical pump has less than 10hp parasitic power loss, so the advantages outweigh the disadvantages of an electric pump. Better aftermarket designs only take 5-7hp at ~6000 rpm, so there is not much to be gained by switching to an electric pump. You ll pay over $300 for a decent electric pump that is truly streetable, so you need to decide if a 10hp gain is worth the cost. Some pumps claim a 15-20hp increase by eliminating parasitic power loss. A 20hp increase is very unlikely unless your stock pump had bad bearings or the impeller was eroded to the point that it was causing detonation problems forcing you to retard time a few degrees. If you are replacing a good stock pump, you very rarely see more than a 10 hp increase, probably closer to 5-7hp. Saying that they eliminate parasitic power loss is untrue in a street car that uses an alternator. The power has to come from somewhere and that somewhere is the alternator. In reality, electric pumps have more parasitic power loss for a given flow rate and I will explain why. Mechanical energy drives the pump; this can be taken right off the crank with a belt as is done with a mechanical pump. In the case of an electric pump, the power comes from the alternator, which is driven from the crank. Rather than the crank directly driving the pump, the mechanical energy is changed into electrical energy by the alternator, then the electric motor on the pump changes it back into mechanical energy to turn the pump. Since electric motors and alternator are not 100% efficient, power is lost in this process. So why do we see a little more power when switching to an electric pump when they are less efficient? The answer is simple; they do not pump as much. The average electric pump flows under 35gph, where the average mechanical pump flows twice as much. If an electric pump flows enough for the application, it can be an advantage. The biggest advantage of an electric pump is that it can be left running with the engine off to cool it off better between rounds. If you have a really good electric pump that can out flow the stock pump at low engine speeds, then you ll have a big advantage in stop and go traffic. I have yet to see any electric pump out flow a stock pump at higher rpm. If you plan doing any road racing, where the rpm stays up for extended periods, a mechanical pump is the only choice. Drag racing is a different animal altogether. You may only see 9 seconds of wide open throttle, heat may build quickly, but an electric pump and fans can run between rounds to cool everything down. Remember, not everything you see on race cars means it s better for your street car. COOLANT FLOW Different coolants require different minimum flow rates, but contrary to popular belief, you cannot make the coolant flow too fast. This rumor was started because people removed the thermostat to gain flow, because they had an over heating problem, and it only aggravated the problem. The real reason they ran into problems is that removing the thermostat also removes the restriction that builds pressure in the engine, so they gained flow, but reduced the boiling point of the coolant in the block. Running a higher flow thermostat and a higher volume pump to maintain pressure, will give no such problems. If you think about it, making the coolant flow twice as fast will also make it flow though the engine twice as often, so there will be more even temperature across the engine. There has been, and still is, the rumor that of the coolant flows too fast, it will not have time to pick up heat. That is nonsense, as long as there is coolant contact a surface, the rate of heat transfer will be the same. Coolant that flows twice as fast also flows through the block twice as often. BASIC FLOW MODIFICATIONS Most stock systems on a V type engine will have a common outlet for both banks. The outlets of each bank flows directly at each other than must take a 90 turn to return to the radiator. If one side gets hotter (which is sure to happen) the pressure of that side will increase. The increased pressure will increase flow in the hotter bank and decrease flow in the cooler one. The faster moving coolant will cool the hot bank better and the slower moving coolant picks up more heat in the colder side. As you can see, the hot side is getting cooled and the cooler side is heating up. This happens until the banks reverse; the side that was cooler is now hotter and has more pressure. The cyclic flow will continue until the 4

5 engine is shut off. Smokey Yunick was the first to do studies on the cyclic flow and traced the problem to the outlet. By tapping the front of the heads, and bringing the coolant together in a Y eliminated the cycling. RADICAL MODIFICATIONS To truly equalize temperatures throughout the engine is not possible with today s technology, but we can improve the situation some. To get the best results you must start fresh and build totally custom cooling system. The first step is to tap off the pump and put coolant to the back of the block so the coolant enters at both ends. This helps equalize the cylinder temperatures, but the heads will still be hotter toward the front. To equalize the head temperatures you must tap outlets at the back of the heads so that all the coolant does not have to pass the front combustion chambers. To further equalize, you can tap inlets and outlets in the center of the block and heads also. At that point the coolant will be flowing basically from bottom to top and is about the best you will get without reversing the flow. REVERSE FLOW SYSTEMS As I said earlier, it makes sense to put the coolest coolant to the hottest parts first to bring the temperatures down as much as possible, the already heated coolant can help bring the temperatures of the coolest parts higher and make everything more even. To do this the coolant must flow in reverse (compared to most systems). The problem with reverse flow systems is that the pump tends to cavitate easier (even with a good aftermarket pump). To limit cavitation, a higher boiling point of the coolant helps and so does a higher system pressure. 5

6 COOLING FANS STOCK CLUTCH FANS The stock clutch fan is often tossed in favor of an electric fan. In this can go two ways, often the wrong way. There are a lot of cheap electric fans on the market that are junk. A factory clutch fan and shroud flows more air than most electric fans. I have seen many people go out and buy electric fans because they had over heating problems. Only to find out that the problem is no better or even worse. The real issue is probably an inadequate cooling system or a poor choice of electric fan. If the problem is a clogged or too small radiator, buying a new fan is not a solution. I have no problem with using electric fans, but they are not all created equal. If your cooling system was borderline adequate before, a crappy electric fan will make it worse. If you are having an over heating problem, don't even consider an electric fan until you find the cause of the problem. If the fan is the problem, make sure the electric fan you choose can outflow the original. For every quality electric fan on the market, you ll find about 10 pieces of junk that make the same claims. FLEX FANS My opinion of flex fans is, they are next to worthless. They can be better than a solid (no clutch) stock fan, and that is the only good thing I have to say about them. They are noisy, and offer little to no benefit over the factory clutch fan. They claim to move a lot of air at low-speeds and flatten out at high speeds to cause little drag. The air hitting the blades is what flattens them out and that takes power to do, so they must have some drag. Maybe not a lot, but certainly more than a clutch fan that is near freewheeling. I personally just do not like them. Flex fans are popular in certain race classes that require an engine driven fan due to the fact that they are light and can take very high rpm. They were popular for a while on the street, probably because they are so cheap and people always insist on buying the "race" parts for a street car. It is very important to do research on these kinds of parts. Race cars do not always use parts because they are the best parts; they are usually the best for what is allowed by the rules in that class. Some of the newer designs of flex fans have the blades curved toward the rotation, this looks like a better design because centrifugal force will assist in flattening out the blades and they should reduce drag over other types of flex fans. I have not had any experience with them, so I can't say for sure, but they appear to be a better design. Actual testing will tell for sure, but I have not personally tried one. ELECTRIC FANS Electric fans can offer many advantages. They are compact, which can really helps when there are space limitations. They are reliable and simple, so it can make for a clean neat installation. There are some good fans out there that will outflow most belt driven fans, but they are not cheap. Although considering the money you have in your engine, paying 2-3 times as much for a good fan is cheap insurance. Another benefit of electric fans is the ability to control them however you want. My ECU for the injection system controls my fans and I can over ride that with a switch in the car to keep the fans on or shut them off if I want. Many aftermarket companies also make thermal switches to control fans. When you add and electric fan, there is always the option of pushing or pulling air through the radiator. So which is best? For the most part, the pulling air through the radiator works better. It is not a question of the fan being more efficient as a puller, if the fan was totally sealed to the radiator so there was no leakage, the pusher would be the ticket, but even with a shroud, 6

7 there is some leakage. A fan does not just flow air through itself straight. A fan spins and causes the air to spin as well. Centrifugal force throws air outward all along the fan as well, but the intake side of the fan is pretty much limited to the area of the fan. When the fan is in front of the radiator, a lot of air goes thrown out and never makes it through the radiator at all. So when you compare total air moved, with a pusher, less makes it through the radiator than the same fan as a puller. A shroud really helps with a pusher, so I recommend a shroud on all pusher fans. Curving the blades toward the direction of rotation like the new flex fan designs might help electric fans as pushers. The curved blades could cup the air and limit the amount thrown outward by centrifugal force. This is just a theory though, some experimenting would tell for sure. One more downfall for the pusher is that it s right in the way of the incoming air, blocking the path to the radiator. In general, a pusher is only about 80% as efficient as the same fan as a puller. FAN SHROUDS To keep is simple; a shroud should be used with any fan to get the most out of it. Any cooling system will benefit a properly designed shroud to pull air through the radiator more evenly at low speeds. A shroud could restrict airflow at high speeds, a simple and effective solution is to cut holes in the shroud and cover them with rubber flaps. At high speeds, air can push the flaps open for additional airflow. Picture a fan with no shroud, there are large portions of the radiator that do not get covered by the fan. This means that if the fan is the only thing moving the air (low vehicle speeds), the radiator is only partially getting used. The areas of the radiator where no air is getting pulled through dissipate very little heat. Without a shroud, a large amount of air can be pulled in from the back side of the radiator, never passing through the radiator. In short, use a shroud. 7

8 RADIATORS COPPER BRASS VS. ALUMINUM Everyone always want s to know which is better. Well, that is not an easy question. If both were made from the same thickness material and has the exact same design, the answer would clearly be copper/brass simply because both copper and brass conduct heat faster than aluminum. A copper brass construction has its downfalls. The tubes, fins and tanks are soldered in place. If the solder used cannot conduct head as well as the copper tubes and fins, it will hurt efficiency. Another thing that hurts efficiency is paint. Too many people use what ever black paint they have laying around and never think about how well that paint conducts heat. Only use radiator paint to paint a radiator. Aluminum radiators are usually left bare aluminum. There are companies out there making quality copper brass radiators that are silver soldered together that will cool just as good as or better than many equally sized aluminum radiators. Aluminum radiators also have some advantages. They are lighter, a typical aluminum radiator can weight lbs less than a copper brass one the same size and thickness. This is not as much as you would think because aluminum radiators are made from thicker material due to the fact that thicker metal can dissipate heat faster. Aluminum does not conduct heat as fast as copper, so thicker material is used to improve heat dissipation. You will see many aluminum radiator companies offer two row radiators, usually 1 or 1 ¼ wide rows. An average aluminum core with two 1 rows is about as wide as a for row copper brass core. The advantage is that there is more fin to tube contact to help transfer heat. As you can see, there is a lot more to choosing a radiator than the material that it s made from. My personal preference is an aluminum radiator, providing is it a quality piece. They are better looking, lighter (even a small weight loss is still a weight loss), and they work just fine from mild to wild engines. I have used Griffin and Be Cool radiators with great success in some pretty radical street cars. CROSS FLOW VS. DOWN FLOW The main advantage that will steer you t toward one or the other is packaging. Vertical cores place the tanks on the top and bottom, making the radiator taller. This works fine for many older cars with narrow tall grills. Most modern cars have more room from side to side, making cross flow radiators more practical. One positive aspect of the cross flow design is that the radiator cap is on the lower pressure side of the radiator. There should not be much pressure drop across the radiator unless it s restrictive, however, so this is not a huge advantage. In most properly set up cooling systems, the thermostat is the highest restriction point, which causes higher pressure in the engine, than the radiator. SINGLE VS. DUAL PASS There is always the question of which is better. My opinion is to go with what fits best. A dual pass will have the inlet and outlet on the same side, where a single pass will have one on the bottom and one on the top on the opposite side. In theory, a single pass is more efficient, but not by much. A single pass radiator allows all cooling tubes to get the hottest coolant all at the same time. The downfall is that velocity is cut in ½ compared to a dual pass, which hurts turbulence and heat transfer rate. A dual pass keeps velocity up, but only ½ of the radiator get the hottest coolant, as the coolant passes 8

9 through the radiator the second time, it is not as hot and transfers heat at a slower rate. The bottom line here is that there is an up and down side to each. The small overall advantage goes to the single pass for most applications, but there are applications where a dual pass can perform better. The advantage either way is only a few percent, so use what fits the best. Most of the time, a single pass will fit the best because of the factory inlet and outlet positions. 9